Chakraborty, S., Fu, R., Rosenfeld, D. & Massie, S.T. The influence of aerosols and meteorological conditions on the total rain volume.
Geophysical Research Letter 45, 23, 13009-13106 (2018).
AbstractThis study provides an observational assessment of the variations of the total rain volume (TRV) with aerosols through the entire lifetime of mesoscale convective systems (MCSs) over tropics. Using 70,000 MCSs' samples, we show that TRV increases with aerosols from clean to moderately heavy polluted conditions (aerosol optical depth [AOD] similar to 0.0-0.4). TRV decreases when AOD exceeds 0.5. The TRV change with AOD is strongest under favorable meteorological conditions, such as high total precipitable water (45-75 kg/m(2)), high convective available potential energy (1,200-2,400 J/kg), and intermediate vertical wind shear (9-21 x 10(-4)/s). TRV of MCSs increases from 2 to 4 km(3) (rain depth similar to 20-40 mm) when AOD < 0.15 or > 0.5, to more than 12 km(3) (similar to 120 mm) when 0.2 < AOD < 0.4 under above the mentioned optimal meteorological conditions. The basic response of TRV to aerosol concentrations is similar under all the meteorological conditions and during all stages of the MCS lifecycle.
Plain language summary Mesoscale convective systems (MCSs) contribute to the largest fraction of global rainfall and are often responsible for devastating flood events. It has long been hypothesized that aerosols can enhance rainfall of MCSs by suppressing rainfall during the early stage of the convection, enabling more cloud droplets to rise to higher altitude and so freeze. Freezing releases more latent heat, which drives strong rising motion and so enables formation of large hydrometeors for heavy rainfall. Thus, it is central to evaluate rainfall changes with aerosols through the entire lifetime of the MCCs. This work provides a first observational assessment of the variation of the total rain generated by MCSs through their lifetime with ambient aerosols, under various ambient meteorological conditions over the global tropical continents. Our results show that aerosols have a strong invigoration effect on MCSs' total rainfall volume. Total rainfall volume increases as AOD increases up to 0.4 and decreases as AOD increases beyond 0.5. Such effects are similar throughout different phases of their convective lifecycle and under various meteorological conditions.
chakraborty_et_al-2018-geophysical_research_letters.pdf Ren, D., et al. Impacts of climate warming on maximum aviation payloads.
Climate Dynamics (2018).
AbstractThe increasing importance of aviation activities in modern life coincides with a steady warming climate. However, the effect of climate warming on maximum aircraft carrying capacity or payload has been unclear. Here we clarify this issue using primary atmospheric parameters from 27 fully coupled climate models from the Coupled Model Inter-comparison Project 5 (CMIP5) archive, utilizing the direct proportionality of near-surface air density (NSAD) to maximum take-off total weight (MTOW). Historical (twentieth century) runs of these climate models showed high credibility in reproducing the reanalysis period (1950–2015) of NSAD. In particular, the model simulated trends in NSAD are highly aligned with the reanalysis values. This reduction in NSAD is a first order global signal, just as is the warming itself, that continues into the future. To examine the statistical significance of the density reduction, a t-test was performed for two 20-year periods 75 years apart (2080–2100 vs. 2005–2025), using the Representative Concentration Pathways (RCP) 8.5 emission scenario of the Intergovernmental Panel on Climate Change (IPCC). Most continental areas easily passed the test at a P-value of 0.05. These future changes of NSAD will likely have significant economic impacts on the aviation industry. For these two 20-year periods that we examined, the most extreme changes are in the Northern hemisphere in high latitudes, i.e., a 5% decrease in MTOW, or ~8.5–19% (aircraft-dependent) reduction in payload. The global average change is about 1%. For the busy North Atlantic Corridor (NAC), the reduction in MTOW is generally greater than 1% and that of payload several times larger.
ren_et_al-2019-Climate_Dynamics.pdf Chakraborty, S., Schiro, K.A., Fu, R. & Neelin, J.D. On the role of aerosols, humidity, and vertical wind shear in the transition of shallow to deep convection at the Green Ocean Amazon 2014/5 site.
Atmospheric Chemistry and Physics 18, 11135-11148 (2018).
Abstract
The preconditioning of the atmosphere for a shallow-to-deep convective transition during the dry-to-wet season transition period (August–November) is investigated using Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) GoAmazon2014/5 campaign data from March 2014 to November 2015 in Manacapuru, Brazil. In comparison to conditions observed prior to shallow convection, anomalously high humidity in the free troposphere and boundary layer is observed prior to a shallow-to-deep convection transition. An entraining plume model, which captures this leading dependence on lower tropospheric moisture, is employed to study indirect thermodynamic effects associated with vertical wind shear (VWS) and cloud condensation nuclei (CCN) concentration on preconvective conditions. The shallow-to-deep convective transition primarily depends on humidity, especially that from the free troposphere, which tends to increase plume buoyancy. Conditions preceding deep convection are associated with high relative humidity, and low-to-moderate CCN concentration (less than the 67th percentile, 1274 cm−3 ). VWS, however, shows little relation to moisture and plume buoyancy. Buoyancy estimates suggest that the latent heat release due to freezing is important to deep convective growth under all conditions analyzed, consistent with potential pathways for aerosol effects, even in the presence of a strong entrainment. Shallow-only convective growth, however, shows an association with a strong (weak) low (deep) level VWS and with higher CCN concentration.
Chakraborty_et_al-2018-Atmospheric_Chemistry_and_Physics.pdf Zhuang, Y., Fu, R. & Wang, H. How do environmental conditions influence vertical buoyancy structure and shallow-to-deep convection transition across different climate regimes?.
Journal of the Atmospheric Sciences 75, 6, 1909-1932 (2018).
Abstract
We developed an entraining parcel approach that partitions parcel buoyancy into contributions from different processes, e.g. adiabatic cooling, condensation, freezing, and entrainment. Applying this method to research quality radiosonde profiles provided by the Atmospheric Radiation Program (ARM) at six sites, we evaluated how atmospheric thermodynamic conditions and entrainment influence various physical processes that determine the vertical buoyancy structure across different climate regimes as represented by these sites. The differences of morning buoyancy profiles between the deep convection/transition cases (DC) and shallow convection/non-transition cases (SC) were used to assess pre-conditions important for shallow-to-deep convection transition. Our results show that for continental sites such as the U.S. Southern Great Plains (SGP) and the West-Central Africa, surface condition alone is enough to account for the buoyancy difference between DC and SC cases, although entrainment further enhances the buoyancy difference at SGP. For oceanic sites in the Tropical West Pacific, humidity dilution in the lower-to-mid free troposphere (~1-6km) and temperature mixing in the mid-to-upper troposphere (>4km) have the most important influences on the buoyancy difference between DC and SC cases. For the humid Central Amazon region, entrainment in both the boundary layer and the lower free troposphere (~0-4km) have significant contributions to the buoyancy difference; the upper tropospheric influence seems unimportant. In addition, the integral of the condensation term, which represents the parcel's ability to transform available water vapor into heat through condensation, provides a better discrimination between DC and SC cases than the integral of buoyancy or the Convective Available Potential Energy (CAPE).
Zhuang_et_al-2018-Journal_of_the_Atmospheric_Sciences.pdf Zhao, B., et al. Impact of aerosols on ice crystal size.
Atmospheric Chemistry and Physics 18, 2, 1065-1078 (2018).
AbstractThe interactions between aerosols and ice clouds represent one of the largest uncertainties in global radiative forcing from pre-industrial time to the present. In particular, the impact of aerosols on ice crystal effective radius (Rei), which is a key parameter determining ice clouds’ net radiative effect, is highly uncertain due to limited and conflicting observational evidence. Here we investigate the effects of aerosols on Rei under different meteorological conditions using 9-year satellite observations. We find that the responses of Rei to aerosol loadings are modulated by water vapor amount in conjunction with several other meteorological parameters. While there is a significant negative correlation between Rei and aerosol loading in moist conditions, consistent with the “Twomey effect” for liquid clouds, a strong positive correlation between the two occurs in dry conditions. Simulations based on a cloud parcel model suggest that water vapor modulates the relative importance of different ice nucleation modes, leading to the opposite aerosol impacts between moist and dry conditions. When ice clouds are decomposed into those generated from deep convection and formed in situ, the water vapor modulation remains in effect for both ice cloud types, although the sensitivities of Rei to aerosols differ noticeably between them due to distinct formation mechanisms. The water vapor modulation can largely explain the difference in the responses of Rei to aerosol loadings in various seasons. A proper representation of the water vapor modulation is essential for an accurate estimate of aerosol–cloud radiative forcing produced by ice clouds.
zhao_et_al-2018-Atmospheric_Chemistry_and_Physics.pdf